Many devices for use in optical communication networks utilize liquid crystal (LC) elements for performing signal modulation functions, generally by means of changing the phase between the ordinary and the extraordinary components of the optical beam transmitted through the element, and hence the polarization direction of the beam, which is generally input with a known polarization. In the prior art there are described numerous examples of such applications, including for instance, those described in PCT Application No. PCT/IL/02/00511 for Wavelength Selective Optical Switch, and in PCT Application No. PCT/IL/02/00188 for Fiber Optical Attenuator, published as WIPO document WO 02071133, and in PCT Application No. PCT/IL/02/00187 for Dynamic Gain Equalizer, published as WIPO document WO 02071660, and in PCT Application No. PCT/IL/02/00167 for Fiber Optical Gain Equalizer, published as WIPO document WO 03009054, all of which are incorporated herein by reference, each in its entirety.
The modulation efficiency of such devices, and especially, the blocking efficiency of such devices used as optical switches, is dependent on the generation of accurate phase differences between the components of the processed optical signals. In particular, it is often necessary to generate an exact phase difference between two components of an optical beam, in order to provide a desired polarization rotation and signal blocking. Any deviation from this phase shift results in degradation in the performance of the device.
In birefringent media, such as in liquid crystal devices, the phase shift between the ordinary and extraordinary beams generated in passage through a pixel of the element is dependent on the ordinary and extraordinary refractive indices respectively no and ne, and is given by the expression:Δφ=Δn·d·2π/λ  (1)where:                d is the path length through the birefringent medium, generally the liquid crystal thickness,        λ is the wavelength of the light passing through the pixel, and        Δn=(no−ne) is the difference between the ordinary and the extraordinary refractive indices of the liquid crystal material, arising from the birefringence of the material. The value of the phase shift Δφ is known as the birefringence, B, of the material of the pixel, because of the origin of the phase shift, though the term birefringence is also sometimes used for the value of Δn·d only.        
The difference Δn can be positive or negative, depending on the type of liquid crystal material used. For a negative nematic LCD, no>ne, for a positive nematic LCD, ne>no. In normal use, the phase shift through any pixel of the liquid crystal element can be changed by varying the applied drive voltage across that pixel, which is operative to change the value of ne as a function of the applied voltage. Many types of devices are in existence using these principles.
It is apparent from equation (1) that the phase shift generated is directly dependent on the wavelength λ, and as the wavelength changes, the phase shift also changes. Consequently, each particular wavelength being switched requires a different applied voltage to ensure that the desired phase difference, for instance of exactly π, is generated for that wavelength. Since in general, in wavelength dispersed applications, the optical channel for each particular wavelength passes through a separate, spatially given pixel, this is readily achieved by ensuring that each pixel has the correct switching voltage applied to provide the desired phase shift, generally exactly π, for that particular wavelength. A look-up table of the required switching voltages as a function of wavelength can be stored in the device control system. So long as environmental conditions remain constant, the values of the phase shift Δφ between pixels associated with different wavelengths can be related by means of the linear dependence on wavelength shown in equation (1).
However, this simple picture is complicated by the fact that the values of no and ne are not generally singly-defined functions, as would appear to be from the simple formulation of equation (1), but also vary with ambient conditions, such as the temperature of the material. This variation in itself could also be accommodated, by providing environmental stabilization of the optical switching device, such as by means of a built-in thermo-electric cooler (TEC) or a heater. The control input to such a TEC or heater may be obtained by measurement of the temperature of the phase shifting element, such as by incorporating a thermistor or a thermocouple. Because of the size of the complete device, this solution, however, requires a comparatively large thermoelectric cooler or a comparatively large heater in order to maintain the desired temperature. This solution is therefore complex, and requires increased power consumption to drive the temperature stabilizing device, especially for a TEC, thus increasing the cost of what should be an otherwise simple device.
However, even the above description of the dependence of the phase shift from equation (1) on wavelength and environmental temperature is incomplete, since it is known that no and ne themselves also vary with the wavelength of the light passing through the medium. The resulting phase shift, as a function of the voltage applied to a specific pixel, thus becomes a more complex function of temperature and wavelength, and even more so because there is also a voltage effect on ne as well. Equation (1) should thus be more fully written as:Δφ=[no(T,λ)−ne(T,λ,V)]·d·2π/λ  (2)where the functional forms of ne and ne are generally different. Calculation of the correct voltages for each pixel, taking into account the functional interaction of all of the various elements of equation (2), by means of predetermined corrections for all envisaged conditions, thus becomes a complex procedure.
There therefore exists a need for a simply applied method of providing environmental compensation of optical devices based on birefringent phase shifting properties, such that the voltage required for obtaining a specific phase shift through any pixel, generally that required for maximum blocking or for maximum transmission, can be determined, even when the functional dependence of the refractive indices of the birefringent material on environmental conditions are not known.
The disclosures of each of the publications mentioned in this section and in other sections of the specification, are hereby incorporated by reference, each in its entirety.